When values of birefingence are very high, the property is termed giant birefringence which more generically is called giant optical anisotropy. Values for giant birefringence exceed 0.3. Much bigger numbers (over 2.0) are termed "colossal birefringence". These are achieved using nanostructures. Some oxides, for example

borate A borate is any of a range of boron oxyanions, anions containing boron and oxygen, such as orthoborate , metaborate , or tetraborate ; or any salt of such anions, such as sodium metaborate, and borax . The name also refers to esters of su ...

iodate An iodate is the polyatomic anion with the formula . It is the most common form of iodine in nature, as it comprises the major iodine-containing ores. Iodate salts are often colorless. They are the salts of iodic acid. Structure Iodate is pyra ...

can have high birefringence. Also compounds containing C=O bonds have higher levels. These include

oxalate Oxalate (systematic IUPAC name: ethanedioate) is an anion with the chemical formula . This dianion is colorless. It occurs naturally, including in some foods. It forms a variety of salts, for example sodium oxalate (), and several esters such as ...

s, squarates and cyanurates. One trade-off is with

band gap In solid-state physics and solid-state chemistry, a band gap, also called a bandgap or energy gap, is an energy range in a solid where no electronic states exist. In graphs of the electronic band structure of solids, the band gap refers to t ...

. If the band gap is small, then the material is not transparent to visible light, but can be transparent for infrared. Chalgogenides may have high birefringence, but only in the infrared. Halide

perovskites A perovskite is a crystalline material of formula ABX3 with a crystal structure similar to that of Perovskite, the mineral perovskite, this latter consisting of calcium titanium oxide (CaTiO3). The mineral was first discovered in the Ural Moun ...

such as CsPbBr_xCl_3−x have fairly high birefringence that varies significantly in the optical spectrum. Some transition metal oxyhalides: MoOCl₄, WOCl₄, have birefringence in the giant category and MoO₂Br₂, WOBr₄, NbOBr₂, and NbOI₂ are predicted to have birefringence over 0.6 at 1065 nm.

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References

{{Reflist Optical phenomena